Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is typically performed in the presence of a carbonylation catalyst and results in the formation of compounds, for example, aldehydes, which have one or more carbon atoms in their molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced in the so-called oxo process by hydroformylation of commercial C.sub.6 -C.sub.12 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields the corresponding C.sub.7 -C.sub.13 saturated alcohols. The oxo process is the commercial application of the hydroformylation reaction for making higher aldehydes and alcohols from olefins. The crude product of the hydroformylation reaction will contain catalyst, aldehydes, alcohols, unreacted olefin feed, synthesis gas and by-products.
A variety of transition metals catalyze the hydroformylation reaction, but only cobalt and rhodium carbonyl complexes are used in commercial oxo plants. The reaction is highly exothermic; the heat release is ca 125 kJ/mol (30 kcal/mol). The position of the formyl group in the aldehyde product depends upon the olefin type, the catalyst, the solvent, and the reaction conditions. Reaction conditions have some effect and, with an unmodified cobalt catalyst, the yield of straight chain product from a linear olefin is favored by higher CO partial pressure. In the hydroformylation of terminal olefinic hydrocarbons, the use of a catalyst containing selected complexing ligands, e.g., tertiary phosphines, results in the predominant formation of the normal isomer.
In commercial operation, the aldehyde product usually used as an intermediate which is converted by hydrogenation to an alcohol or by aldolization and hydrogenation to a higher alcohol. The aldol-hydrogenation route is used primarily for the manufacture of 2-ethylhexanol from propylene via n-butyraldehyde.
The hydroformylation reaction is catalyzed homogeneously by carbonyls of Group VIII metals but there are significant differences in their relative activities. Roelen, using a cobalt catalyst, discovered hydroformylation in 1938. Dicobalt octacarbonyl, CO.sub.2 (CO).sub.8, which either is introduced directly or formed in situ, is the primary conventional oxo catalyst precursor. Using an unmodified cobalt catalyst, the ratio of linear to branched aldehyde is relatively low.
Much oxo research in the past 25 years has been directed to improving reaction selectivity to the linear product. Introduction of an organophosphine ligand to form a complex, e.g., Co.sub.2 (CO).sub.6 [P(n-C.sub.4 H.sub.9).sub.3 ].sub.2, significantly improves the selectivity to the straight-chain alcohol.
Recent developments of low pressure rhodium catalyst systems have been the subject of a considerable body of patent art and literature, and rhodium-triphenyl phosphine systems have been widely, and successfully, used commercially for the hydroformylation of propylene feedstocks to produce butyraldehyde.
The first commercial oxo process to employ a rhodium-modified catalyst was developed by Union Carbide, Davy Powergas, and Johnson Matthey. In this application, the complexed rhodium catalyst is dissolved in excess ligand and the reaction is run at relatively low pressures and temperatures as compared to a conventional oxo process. The ratio of normal to iso isomers is high relative to conventional oxo processes and so is favored as a process for the production of N-butyraldehyde.
A recent process commercialization has been that of Rhone-Poulenc and Ruhrchemie which produces butyraldehyde from propylene but the ligand is a sulfonated triphenylphosphine and is utilized as a water soluble sodium salt. Turnover rates are less than in the all-organic system, but the normal to iso ratios are high and the catalyst may be separated easily from the reaction product by separation of the aqueous layer containing the catalyst and the organic layer which constitutes the product.
In the formation of linear aldehydes using a ligand-modified rhodium-catalyzed homogenous process, the reactor comprises the rhodium complex catalyst, excess triphenylphosphine and a mixture of product aldehydes and condensation by-products. The product aldehyde may be recovered from the mixture by volatilization directly from the reactor or by distillation in a subsequent step. The catalyst either remains in or is recycled to the reactor. However, the complex catalyst and triphenylphosphine ligand are slowly deactivated and eventually the spent catalyst is removed for recovery of rhodium and reconversion to the active catalyst. This process, although effective for lower molecular weight aldehyde production, is not favored for higher molecular weight aldehydes which are higher boiling, distillation temperatures needed for aldehyde recovery are higher and catalyst deactivation is accelerated.
The aqueous ligand system is also very effective for propylene but higher molecular weight olefin feeds are not sufficiently soluble in the aqueous catalyst medium to allow acceptable rates of aldehyde formation. Thus, although separation of the higher molecular weight aldehyde should be more facile than the all-organic system, the slow rates preclude commercial acceptability.
In some cases, such as where the products of the reaction are relatively high boiling or where the olefin feed is not sufficiently soluble in water to permit satisfactory reaction rates, neither the process where the products are removed from the catalyst by distillation or stripping nor where the products are decanted from an aqueous catalyst solution may be utilized successfully. In such cases, it may be advantageous to utilize an aqueous medium to contain the catalyst and add a surfactant to enhance phase contacting so as to improve rate and selectivity to the desired products. This type of process is called "Phase Transfer Catalysis." However, when the surfactant is added, some carry-over of the noble metal into the organic phase at the conclusion of the process often results.
The present inventors have discovered that when they satisfactorily hydroformylated olefins in the presence of water soluble Group VIII noble metal-ligand complex catalysts using an aqueous-organic medium enhanced by surfactants, the catalyst can be recovered quantitatively from a crude reaction product which includes both an aqueous phase and an organic phase by employing membrane separation either internal or external to the hydroformylation reactor.
It has been known to use membranes to separate catalysts from an aqueous solution. An example is set forth in European Patent No. 0 263 953, published on Aug. 29, 1986 (assigned to Ruhrchemie Aktiengesellschaft), which discloses a process for separating rhodium complex compounds, which contain water-soluble organic phosphines as ligands, from aqueous solutions in which excess phosphine ligand and, if necessary, other components are also dissolved, is characterized by the fact that the aqueous solution is subjected to a membrane separation process. According to this process, volatile organic substances are separated from the solution prior to conducting the membrane separation process. A typical membrane for use in this process is a cellulose acetate membrane. This process only involves the separation of water-soluble ligands and noble metal catalyst from an aqueous solution. As such, this separation process does not pertain to the separation of a water soluble noble metal catalyst and a water soluble ligand from an organic-aqueous emulsion, dispersion or suspension produced from the hydroformylation process.
Another patent which utilizes cellulose acetate, silicone rubber, polyolefin or polyamide membranes in the separation of catalysts from high boiling by-products of the hydroformylation reaction is Great Britain Patent No. 1312076, granted on May 15, 1970. According to this patent the aldehydes produced during the hydroformylation process are continuously withdrawn as an overhead vapor stream. The liquid stream containing the heavy by-products with the catalyst is passed over a membrane wherein approximately 78-94.3% of the catalyst is retained and the heavy by-products permeated. This is an unacceptably low level of catalyst retention which is overcome by the process of the present invention.
In like manner, Great Britain Patent No. 1432561, granted on Mar. 27, 1972, (assigned to Imperial Chemical Industries LTD.) discloses a process for the hydroformylation of olefins which comprises reacting an olefin at elevated temperature and pressure with CO and H.sub.2 in the presence of a compound of a group VIII metal and a biphyllic ligand of a trivalent P, As or Sb to give a crude liquid hydroformylation product containing an aldehyde and/or an alcohol, separating the aldehyde and/or alcohol from the crude product and leaving a liquid, bringing the liquid after separation of the Group VIII metal compound and free from aldehyde and alcohol under reverse osmosis conditions into contact with one side of a silicone rubber semi-permeable membrane in which the polymer chains have been at least partly cross-linked by gamma radiation whereby the liquid retained by the membrane contains a higher concentration of Group VIII metal compounds and/or biphyllic ligand than the original liquid.
In the article by Gosser et al., entitled "Reverse Osmosis in Homogeneous Catalysis," Journal of Molecular Catalysis, Vol. 2 (1977), pp. 253-263, a selectively permeable polyimide membrane was used to separate soluble transition metal complexes from reaction mixtures by reverse osmosis. For example, separation of cobalt and rhodium complexes from hydroformylation products of 1-pentene. That is, a solution of 0.50 g of RhH(CO)(PPh.sub.3).sub.3 in 40 ml of benzene and 10 ml of 1-pentene was stirred at 50.degree. C. with a CO/H.sub.2 mixture at ca. 4 atm pressure until no further pressure drop occurred. The pentene was completely converted to aldehydes according to proton nmr analysis. The solution was permeated through a polyimide membrane under 68 atm nitrogen pressure. The permeate (4.5 g passed in 2 min.) showed only 9% of the original rhodium concentration by X-ray fluorescence.
The permeation rate of rhodium as set forth above, i.e., 9%, is considered unacceptable. The rhodium catalyst should be retained in an amount of greater than 99.5% to be a commercially feasible process.
Another example of the use of membranes to separate metal catalysts from hydroformylation products is set forth in Dutch Patent No. 8700881, published on Nov. 1, 1988. The method disclosed therein relates to one which improves the efficiency of membrane separation of hydroformylation products from expensive organometallic catalyst containing reaction mixtures. In Dutch Patent No. 87/881 a polydimethylsiloxane membrane having a thickness of 7 microns applied to a Teflon.RTM. support was used in the separation of a reaction mixture containing C.sub.9 -C.sub.15 alcohols, a homogeneous catalyst system comprising an organometallic complex of a transition metal from Group VIII or VIIa or Va of the Periodic Table, e.g., a tricarbonyl(triphenylphosphine) cobalt catalyst, and 40% low-viscosity lubricating oil (an antiswelling or de-swelling agent). At a flow of 133 kg/m.sup.2 -day, the cobalt contents in the feed, retentate, and permeate were 600, 910, and 18 ppm, verses 840, 1930, and 160 ppm, respectively, for a mixture without the de-swelling agent. This process is directed to the separation of product from a reaction mixture containing a homogeneous catalyst system by means of a membrane, whereas the present invention is directed to a heterogeneous catalyst system comprising both an organic and an aqueous layer. The ligands disclosed in Dutch Patent No. 8700881 are all organic soluble ligands, e.g., triphenylphosphine, tri-n-alkylphosphine or acetyl acetonate, whereas those used in the present invention are water soluble ligands. Critical to the process of Dutch Patent No. 8700881 is the addition of a de-swelling agent to the reaction mixture which assists in the separation of the products from the reaction mixture.
Each of the aforementioned processes for removing metal catalysts from crude hydroformylation reaction products are both costly in terms of unrecovered catalyst and, as such, would require further expensive treatment of the streams to recover catalyst.
The present invention provides a ligand and membrane combination which allows for the retention of over 99.5% of the noble metal catalyst from the hydroformylation reaction product which is passed over the membrane. Moreover, the hydrophobic membrane used in accordance with the process of the present invention remains thermally and hydrolytically stable during separation.
The present inventors have been able to demonstrate that an aqueous-organic-catalyst mixture can be separated from the crude hydroformylation product mixture using a hydrophobic membrane and a perstracting organic solvent. This novel process permits the organic products to permeate through the membrane, while retaining the rhodium catalyst and all other water soluble components.
The present invention also provides many additional advantages which shall become apparent as described below.